The present invention relates to a heat-resistant yellow pigment particles, and more particularly, to heat-resistant yellow pigment particles which are free from contamination of harmful metals such as Pb, Cr, Cd, etc., show a clear yellow color, are excellent in heat resistance, chemical resistance and light resistance, and have a low specific gravity, and which comprises composite metal oxide particles comprising Fe, Ti and Li among which at least a part of Fe and/or Ti may be substituted with Al and/or Mg, and comprising a crystal phase having a pseudobrookite-type structure.
In addition, the heat-resistant yellow pigment particles according to the present invention, are useful as a yellow pigment for yellow paints used for road-signs, etc., because of clear yellow color and excellent heat resistance thereof.
As conventional inorganic yellow pigments, there have been extensively used chrome yellow (PbCrO.sub.4), cadmium yellow (CdS.nZnS) or the like, since these pigments can exhibit an extremely clear yellow color. However, the chrome yellow comprises Cr and contains Pb which are harmful metals, and the cadmium yellow comprises Cd which is harmful metals. Therefore, for example, in the case where these pigments are used as yellow paints for road-signs, the harmful metals such as Pb, Cr and Cd tend to be eluted from coating films of these yellow paints due to corrosion, etc., resulting in causing environmental pollution. Accordingly, there arises a problem concerning safety.
On the other hand, many of known organic pigments which can show a clear yellow color, are harmful and have a low biodegradability in natural environment, so that the use thereof also causes a problem. In recent years, it has been required to use environment-favorable materials. In order to meet such a requirement, there has been a demand for providing inorganic yellow pigments containing no harmful.
Further, goethite (.alpha.-FeOOH) which is known as inorganic yellow pigment showing a clear yellow color, is iron oxide hydroxide and, therefore, has no problem concerning safety. However, goethite undergoes the change in crystal structure and is transformed into red-colored hematite (.alpha.-Fe.sub.2 O.sub.3) when heat-dehydrated at a temperature of not less than about 230.degree. C. Therefore, the goethite does not show a sufficient heat resistance required for a yellow pigment. Especially, the goethite cannot be used as a yellow pigment for yellow paints used for road-signs which tend to be exposed to more severe environmental conditions. This is because these yellow pigments for yellow paints used for road-signs are required to have a heat resistance which is capable of inhibiting occurrence of discoloration or color degradation even under a higher temperature condition of at least about 250.degree. C.
In the case where the yellow pigment is used as a color pigment of the yellow paint, e.g., for road-signs or the like, the yellow paint is frequently exposed to wind or rain for a long period of time. Accordingly, the yellow pigment used is also required to have not only an acid resistance so as not to be readily dissolved in acid rains, but also an alkali resistance, i.e., required to show a chemical resistance.
Also, rutile-type titanium dioxide (TiO.sub.2) have been extensively used as a white pigment for mixing of colors. It is known that the rutile-type titanium oxide has a photocatalytic effect. For this reason, when a pigment component contains the rutile-type titanium oxide, it is likely that a binder resin or the like used therewith is disadvantageously decomposed due to the photocatalytic effect thereof, resulting in deterioration of the light resistance. Therefore, the yellow pigment has been required to show a good light resistance.
In general, since inorganic pigments have a large true specific gravity, many of these inorganic pigments show a poor dispersion stability in paints. This is because the true specific gravity of the binder resin contained in the paint is about 1.5 at most, while the true specific gravity of the inorganic pigment dispersed in the paint is larger, e.g., 4.23 in the case of titanium dioxide particles and 5 in the case of magnetite particles, etc. For this reason, it has been demanded to provide a yellow pigment having a low specific gravity.
Under the above-mentioned circumstances, it has now been desired to provide inorganic yellow pigments which contain no harmful metals such as Pb, Cr or Cd, show a clear yellow color, are excellent in heat resistance, chemical resistance and light resistance, and have a low specific gravity.
Meanwhile, iron oxide and titanium oxide are oxides which are unharmful to human bodies, and can form composite oxides having various composition ratios. However, it is known that among typical ones of these composite oxides, for example, FeTiO.sub.3 (ilmenite) shows a black color and Fe.sub.2 TiO.sub.5 (pseudobrookite) shows a brown color. On the other hand, it has been reported that colored pigments having various color tones are obtained by first preparing Fe.sub.2 TiO.sub.5 or the like from anatase-type titanium oxide (TiO.sub.2) and hematite (.alpha.-Fe.sub.2 O.sub.3) as raw materials, and then mixing these minerals with white-colored rutile-type TiO.sub.2 ("Coloring Material", 57, pp. 652 to 659 (1984); Japanese Patent Publication (KOKOKU) No. 49-46909(1974)).
Hitherto, there have been various attempts for producing yellow pigments composed mainly of composite metal oxide particles containing Fe and Ti. For example, there are known an inorganic yellow pigment composed of a solid solution constituted by Fe.sub.2 TiO.sub.5 having a pseudobrookite lattice (pseudobrookite-type structure) and rutile-type or anatase-type titanium dioxide (Japanese Patent Application Laid-Open (KOKAI) No. 50-51128(1975)), and a pigment composed of a titanium oxide/iron oxide composite sol (Japanese Patent Application Laid-Open (KOKAI) No. 8-239223(1996)).
In addition, as pigments composed of composite metal oxide particles containing not only Fe and Ti but also other metal elements, there are known, for example, a yellow inorganic pigment composed of composite metal oxide particles containing titanium, iron and molybdenum (Japanese Patent Application Laid-Open (KOKAI) No. 60-42236(1985)), a yellow pigment constituted by a mixed material having a pseudobrookite-type crystal structure forming a solid solution with Al and a rutile-type crystal structure (Japanese Patent Application Laid-Open (KOKAI) No. 8-73224(1996)), and heat-resistant inorganic pigment particles composed of composite oxide containing Fe, Ti and other metals (Japanese Patent Application Laid-Open (KOKAI) No. 9-221323(1997)).
Further, as pigments composed of composite metal oxide particles containing Fe and Ti, in which alkali metal elements are further incorporated, there are known a pigment containing a metal oxide having a rutile-type or polyrutile-type structure and a metal fluorine compound as main components (Japanese Patent Publication (KOKOKU) No. 37-3460(1962)), and a yellow- to skin-colored pigment obtained by calcining a mixture having a specific composition, composed of an alkali metal compound, a ferric compound and titanium dioxide (Japanese Patent Publication (KOKOKU) No. 3-21580(1991)).
Although it have been desired to provide inorganic yellow pigments which are free from contamination of harmful metals such as Pb, Cr or Cd, show a clear yellow color, are excellent in heat resistance, chemical resistance and light resistance, and have a low specific gravity, the pigments described in the above prior arts are still unsatisfactory.
That is, the pigment described in Japanese Patent Application Laid-Open (KOKAI) No. 50-51128(1975) is composed of a mixture of pseudobrookite (FeTiO.sub.5) and rutile-type or anatase-type titanium dioxide. Thus, the pigment show such a color tone obtained by color-mixing of brown-colored pseudobrookite (FeTiO.sub.5) and white-colored titanium dioxide, but cannot show a clear color.
In the case of the pigment described in Japanese Patent Application Laid-Open (KOKAI) No. 8-239223(1996), since the titanium oxide/iron oxide composite sol is merely a mixture, the pigment does not have a crystal phase having a pseudobrookite-type structure.
The pigment described in Japanese Patent Application Laid-Open (KOKAI) No. 60-42236(1985) contains molybdenum as a heavy metal, and does not have a pseudobrookite-type structure.
The pigment described in Japanese Patent Application Laid-Open (KOKAI) No. 8-73224(1996) has a pseudobrookite-type structure forming a solid solution with Al, but does not contain Li.
The pigment described in Japanese Patent Application Laid-Open (KOKAI) No. 9-221323(1997) contains various metal elements in addition to Fe and Ti, but does not contain Li.
In Example of Japanese Patent Publication (KOKOKU) No. 37-3460(1962), there is described a pigment composed of a composite oxide containing Fe, Ti and Li. However, the composite oxide has a rutile-type or a polyrutile-type structure and is, therefore, deteriorated in light resistance.
As to the yellow pigment particles described in Japanese Patent Publication (KOKOKU) No. 3-21580(1991), the composite oxide containing Fe, Ti and Li is described in Example thereof. However, the composite oxide has a spinel-type structure and, therefore, cannot show a clear yellow color. Further, the pigment particles cannot exhibit a sufficient chemical resistance.
Accordingly, it is a technical subject of the present invention to provide inorganic yellow pigments which are free from contamination of harmful metals such as Pb, Cr or Cd, show a clear yellow color, are excellent in heat resistance, chemical resistance and light resistance, and have a low specific gravity.
As a result of the present inventor's earnest studies, it has been found that by mixing an Fe compound, an Ti compound and an Li compound together at specific ratios, calcining the mixture at a temperature of 700 to 1,000.degree. C., and pulverizing the resultant calcined material into particles, there can be obtained a yellow pigment which can show a clear yellow color, are excellent in heat resistance, chemical resistance and light resistance, and have a low specific gravity. The present invention has been attained on the basis of the finding.